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A pre-load of 0.5.1.0 provides for stable operation because a converging wedge is produced between the bearing journal and the bearing pads.
The variable C. is an installed clearance and is dependent upon the radial pivot position. The variable C is the machine clearance and is fixed for a given bearing. Figure 13-7 shows two pads of a five-pad tilting-pad bearing wherethe pads have been installed such that the pre-load ratio is less thanone， and Pad 2 has a pre-load ratio of 1.0. The solid line in Figure 13-7 represents the position of the journal in the concentric position. The dashed line represents the journal in a position with a load applied to the bottom pads.
From Figure13-12， Pad 1 is operating with a good converging wedge，while Pad 2 is operating with a completely divergingfilm， thus indicating thatthe pad is completely unloaded. Therefore， bearings with pre-load ratios of
1.0 or greater will be operating with some of their pads completelyunloaded，thus reducing the overall stiffness of the bearing and decreasing its stability， since the upper pads do not aid in resisting cross-coupling influences.

Unloaded pads are also subject to flutter， which leads to a phenomenon known as ""leading-edge lock-up.'' Leading-edge lock-up causes the pad to beforced against the shaft， and it is then maintained in that position by the frictional interactionof the shaft and the pad. Therefore， it is of primeimportance that the bearings be designed with pre-load， especially for low-viscosity lubricants. In manycases， manufacturing reasons and the ability to have two-way rotation cause many bearings to be produced without pre-load.
Bearing designs are also affected by the transition of the film from a laminar to a turbulent region. The transition speed (.t) can be computed using the following relationship:
v
.t二 1.57 . 103 ..........

.C3
where:
v二 viscosity of the fluid
.二 diameter (inches)
C二 diametrical clearance (inches)
50
(28) 45 40

(22) 35

30 25

(14)

20 15
(8) 10
5
0

0 2 4 6 8 10 12 14 16 1820 Surface Speedx 1000 (ft/min)
Figure 13-13. .il discharge characteristics.
Turbulence creates more power absorption， thus increasing oil tempera-ture that can lead to severe erosion and fretting problems in bearings. It is desirable to keep the oil discharge temperature below 170 oF (77 o.)， butwith high-speed bearings， this ideal may not be possible. In those cases， it is better to monitor the temperature difference between the oil entering and leaving as shown in Figure 13-13.
Bearing .aterials
In all the time since Issaac Babbitt patented his special alloy in1839， nothing has been developed that encompasses all of its excellent properties as an oil-lubricated bearing surface material. Babbitts have excellent com-patibility and nonscoring characteristics and are outstanding in embedding dirt and conforming to geometric errors in machine construction and oper-ation. Theyare， however， relatively weak in fatigue strength， especially at elevated temperatures and when the babbitt is more than about 0.015 of aninch (.381 mm) thick as seen in Figure 13-14. Ingeneral， the selection of abearing material is always a compromise， and no single composition can include all desirable properties. Babbitts can tolerate momentary rupture ofthe oilfilm， and may well minimize shaft or runner damage in the event of a complete failure. Tin babbitts are more desirable than the lead-based mater-ials， since they have better corrosion resistance， less tendency to pickup onthe shaft orrunner， and are easier to bond to a steel shell.
Application practices suggest a maximum design temperature of about 300 oF (149 o.) for babbitt， and designers will set a limit of about 50 oF (28 o.) less. As temperatures increase， there is a tendency for the metal to
0.045
0.04
(1.0mm)
0.035
　
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